contents

Introduction - 1

History - 2

Overview - 3

Advanced features - 4

Work in progress - 5

One possibility - 6

Conclusion - 7

References

Appendix 1 - blowfish

Appendix 2 - tkspline

4.2 - Fallback to Tcl

Now that using C is less daunting for the average Tcl programmer, "plug-in replacement by C code"  becomes a valid strategy.  But sometimes we can't use a compiled version on a given platform -  e.g. if there is no compiler available on the platform,  or if the C code uses features that are not portable.  In this case it is important to be able to fall back to a pure-Tcl implementation - even if it is slower, or perhaps missing some features. The aim is to always end up with some working code, even if that code is nothing more than a clearly expressed failure explanation.

To see how this is done, let's look again at the first example, and extended the mymath example so that it will fall back to a Tcl implementation if a C compiler isn’t available, or if the compile fails for any reason:

   package provide mymath 1.0
   package require critcl

   proc fallback {} {
      proc ::noop {} {}
      proc ::add {x y} {
           return [expr {$x + $y}]
      }
      proc ::cube {x} {
           return [expr {$x*$x*$x}]
   }
   if {[critcl::scripting]} {
      fallback
   } else {
      critcl::cproc noop {} void {}
      critcl::cproc add {int x int y} int {
         return x + y;
      }
      critcl::cproc cube {int x} int {
           return x * x * x;
      }
      if {[critcl::failed]} {
           fallback
      }
   }

The critcl::failed procedure can be called once within a Critcl script. It forces a compilation of the generated C code and returns true if the compilation (or link) fails. This is typically used at the end of a Critcl script to either fall back to Tcl code and/or to issue a warning message. Note that invoking critcl::failed stops any compiler errors being displayed (they can still be viewed in the Critcl log file under ~/.critcl/platform.

This fallback approach could be useful in projects such as Tcllib[11], which must be guaranteed to work on platforms without a C compiler but that contain modules which could benefit from the speed of a C implementation.

see also

• Critcl Home Page

• Tclkit Home Page

• Starkit Home Page

• Wikit Home Page

• Tclers' Wiki

• Steve's Website

• Jean-Claude's Website

Paper by S. Landers & J.C. Wippler, as presented at Tcl/Tk 2002 conference - see also original PDF.